Several devices for recording X-rays in the form of an electrostatic charge pattern have been disclosed. Basically two types of devices have been disclosed, a first type wherein charge images are generated by image-wise exposing the device to X-rays and a second type wherein an overall charge pattern is generated by an overall exposure to visible light and wherein the X-ray exposure creates a charge image by image-wise discharging the overall charges in the device.
In U.S. Pat. No. 3,069,551 an apparatus according to the first type, for intensifying images is disclosed, wherein two photoconductive layers A and B of different materials are separated by a thin mosaic T electrically conducting in the direction normal to its surface but insulating in the transverse direction. This anisotropic conductivity can be obtained by using a closely packed mosaic of evaporated metal such as is known in the art. The outer sides of the photoconducting layers carry transparent conducting layers F and G across which a D.C. voltage is applied from a source of potential E. In operation, one side of is irradiated continuously with the image radiation of a source S.sub.1 which might be light or X-rays and which does not excite B. Charge is thus allowed to conduct across the layer A and mosaic T and built at the interface between T and B. The second photoconductive layer is scanned simultaneously with a spot of exciting radiation L from a source S.sub.3 which discharges the build-up charge point by point as it moves across the layer B and gives electronic signals to an appropriate amplifier.
In U.S. Pat. No. 4,818,857 another device, according to the first type, is disclosed wherein an electrode is in contact with a layer of photoconductive material which is in close contact with a layer of dielectric material, comprising an electret and wherein said layer of dielectric material is contact with a second electrode. Upon an image-wise X-ray radiation, via the photoconductive layer, charges accumulate image-wise at the interface of the photoconductive layer and the dielectric layer. These charges can then be scanned and converted in an image.
In U.S. Pat. No. 4,508,966 still another device, according to the first type, is disclosed. This device comprises a single photoconductive layer sandwiched between two isolating layers. Each of the isolating layers carries an electrode. For X-ray recording a DC voltage is applied between the two electrodes, and by image-wise exposing the device to X-rays, a charge pattern is formed in the photoconductive layer, which can be read out and converted to a visible image.
In the periodical Research Disclosure, June 1983, item 23027 an energy transducer device, according to the second type, is characterized by :
(1) two contacting photoconductive layers of different spectral sensitivity, PA1 (2) each of said photoconductive layers stands in electrically conductive contact with a different electrically conductive support or layer, PA1 (3) at least one of said electrically conductive layers is transparent to radiation to which its contacting photoconductive layer is sensitive, PA1 (4) the other photoconductive layer can be irradiated through its contacting photoconductive layer or through its contacting support or layer to increase its conductivity, PA1 (5) a direct current (DC) source for applying a voltage difference between said conducting layers or supports, and PA1 (6) an electric-signal-detecting means, e.g. resistor, between one of the poles of said current source and one of said conductive layers or supports. PA1 (A) the overall exposure, i.e. overall exposure, of one of said photoconductive layers to light whereby it becomes uniformly electrically conductive while a DC voltage difference is applied between the electrically conductive supports or layers, PA1 (B) information-wise exposing the other photoconductive layer to electromagnetic radiation, e.g. X-rays, to which it is sensitive and to which the overall exposed photoconductive layer is substantially not sensitive while the support or layer in contact with the information-wise exposed photoconductive layer is connected to a means, e.g. ground, accepting charges leaking through the information-wise exposed photoconductive layer in correspondence with its exposed areas, and PA1 (C) scanning-wise exposing the overall exposed photoconductive layer with light to which it is sensitive and to which the other photoconductive layer is substantially not sensitive while with a current source a DC voltage is applied between said conductive supports or layers having a current signal detecting means, e.g. resistor, connected between one of the poles of said voltage source and one of said conductive supports or layers. PA1 (i) applying a DC potential between a first and a second electrode, said first electrode being transparent to X-rays and being in direct contact with an isolating layer, said isolating layer being in direct contact with a first layer of photoconductive material sensitive to X-rays, said first layer of photoconductive material being in direct contact with a second layer of photoconductive material sensitive to visible light, said second layer of photoconductive material being in direct contact with a layer of isolating material, said layer of isolating material being in direct contact with said second electrode, both said isolating material and said second electrode being transparent to visible light, PA1 (ii) overall exposing said second layer of photoconductive material, through said second electrode and said isolating material by visible light, for accumulating charges at the interface between said two photoconductive layers, PA1 (iii) image-wise exposing said first layer of photoconductive material to X-rays, during said overall exposure of said second photoconductive layer, for forming a charge image at said interface, PA1 (iv) reading said charge image by scanning said second photoconductive layer by visible light.
The method of operating said transducer device comprises the following steps :
Said device is particularly suited for recording an X-ray pattern and transforming it in corresponding signal currents that can be amplified, stored and used in display on CRT-tube or for modulating a writing light beam in the production of a hard copy, e.g. on photographic silver halide emulsion film.
Another device of the second type is disclosed in the periodical Research Disclosure, April 1985, number 252, item 25235. This device comprises a first electrode, transparent to visible light in close contact with a first photoconductive layer sensitive to visible light, but not to X-rays. Said first photoconductive layer is in direct contact with a second photoconductive layer, sensitive to X-rays. Said second layer is in direct contact with a transparent electrode which in turn is in contact with a direct emitting X-ray phosphor. This device is overall exposed to light via said first photoconductive layer to form an overall charge pattern at the interface between the two photoconductive layers. Then upon image-wise X-ray exposure, charges leak away from said interface, said leaking away being helped by the visible light image-wise emitted by the direct emitting X-ray phosphor. The remaining charge image is then read out and converted to a visible image.
It is still desirable to improve X-ray recording devices of the second type, as described above, so that higher speed and a better signal-to-noise ratio (SNR) can be achieved.